Human contraceptives were first tested on animals, and
then later used to control fertility in animals. Starting in the 1960s,
hormonal contraceptives were tested on a large number of animals
including, among others, dogs, lions, deer, horses, rats, and birds.
Delivery of these compounds was comparatively simple for zoo animals where
much of the early research occurred, but difficult for free-ranging
wildlife, because large and frequent doses were necessary, and also
because baits were often avoided. In addition, social behavior was
sometimes adversely affected. A major disadvantage concerned passage of
these steroid hormones through the food chain so that other, non-targeted
species, such as predators, or even humans, might ingest them. By 1987,
data clearly revealed that hormonal contraceptives were causing health
problems in, at least, some animals.

Since the late 1980s, much of the research has been
directed toward immunocontraception, a method whereby an individual animal
is injected with a substance that causes it to produce antibodies that
interfere with a process necessary for reproduction. Many different kinds
of immunocontraceptives are possible. Vaccinations could counteract the
effect of hormones made by the brain, by the pituitary gland, and so
forth. The vaccine that has been most widely tested and developed is PZP
(porcine zona pellucida).

The zona pellucida is the non-cellular membrane that
surrounds the mammalian egg. When the zona pellucida from a pig is
inoculated into an animal of a different species, that animal produces
antibodies that prevent sperm from penetrating it own zona pellucida, thus
preventing fertilization, and reproduction.

There are a number of practical advantages, some of which
translate into ethical advantages, in using PZP. It is, on average,
approximately ninety per cent effective, and does not pass through the
food chain. Since only a small amount of the vaccine is needed, it can be
delivered remotely by a dart, thus avoiding the stress of capture, or
anesthesia. PZP does not significantly affect social behaviour and, so
far, has not been found to cause any serious health problems. It is safe
for pregnant animals, is reversible, and is effective on a broad range of
species. The vaccine itself is very inexpensive. Costs for an
immunocontraceptive program vary largely according to personnel expenses.

In general, then, there are many advantages to PZP
immunocontraception. It is humane for animals, since it causes neither
death nor discomfort. It represents no danger to humans, since it does not
involve guns or razor-tipped arrows. It is effective since there is no
rebound reproduction, a phenomenon that occurs when large numbers of
animals are suddenly removed. Bizarre and often troublesome animal
behavior is avoided, since the young are not orphaned, and social
structure is not affected. Genetic diversity is not lost since it is
reversible. Lethal methods, such as hunting and culling, offer none of
these advantages.

Immunocontraceptives in general, and PZP in particular,
seems to be desirable if it is used as an alternative to culling or
hunting to control wildlife populations, or if it is used to limit the
reproduction of zoo animals so that so-called "surplus" animal are not
born.

Hunters, of course, do not view immunocontraception as
desirable for they fear that its use threatens their "life style." They
are, however, not alone in their criticism of PZP. Some animal rightists
have also raised ethical objections claiming that immunocontraception is
simply one more example of human domination and manipulation of the
natural world, one more way in which we control nature for our own ends,
and with our own welfare as the primary criterion. This criticism is
valid, but it would also apply to our habitual ways of treating animals,
which include shooting, poisoning, gassing, habitat manipulation, and so
on. Seen within this context, immunocontraception appears benign, even if
it does not represent the ideal of non-interference with, and respect for,
all non-human animals.

Another objection concerns the fact that PZP is a
slaughterhouse product made from pig ovaries. It is claimed that
purchasing these ovaries only increases slaughterhouse profits, and
encourages more slaughter. This objection is also valid, but, again, it
can be argued that the profit increase is very small. Furthermore, whether
or not their ovaries were sold, pigs would still be slaughtered for their
meat. Eliminating the use of PZP would not lessen the number of pigs
killed. This objection would be nullified if the ongoing research that is
directed at genetically engineering PZP were successful.

A third, although infrequently heard, objection to PZP is
that the development of this vaccine inevitably leads to more animal
experimentation. Most scientists believe it is a necessary part of the
scientific method to kill and necropsy animals to order to prove that PZP
is harmless. According to this view, the fact that PZP vaccinated horses
lived longer, and were in better condition than those who were not
treated, does not militate against this necessity.

A fourth objection is the ease with which PZP can be
"misused." It is one thing, critics assert, to use PZP as an alternative
to killing, and quite another to control the number of predators to
protect their prey for the benefit of hunters or fishers. Actual instances
of such misuses have already occurred. Denying that they had over-fished,
Canadians wanted to contracept seals because they claimed that the seals
were responsible for the lack of fish. Similarly PZP is being used in
Canada to reduce the number of wolves to prevent them from preying upon
caribou herds that hunters want to exploit.

Scientists continue their research, not only on PZP to
make it effective for a longer period, but also on other kinds of
immunocontraception. Scientists are also working to produce a sterilant
for domestic cats and dogs that is as effective, but easier and less
invasive than surgically spaying and neutering.

Immunocontraception: ethically desirable or the opening of
Pandora's box? The jury is still out.

For more information on immunocontraception and PZP in
particular, see www.pzpinfo.org

[Professor Priscilla Cohn is an internationally known
philosopher. She has a doctorate in philosophy from Bryn Mar College, and
is professor emeritus of Abington College, Pennsylvania State University,
USA. She has written on animals, environmental issues and ethical
problems, as well as on contemporary philosophers and the history of
philosophy, publishing both in English and Spanish. Among her books are
Etica aplicada (Applied Ethics) with Jose Ferrater Mora, and, more
recently, Ethics and Wildlife (Edwin Mellen Press, 1999). She has lectured
on five continents, including Spain where Queen Sofia attended her summer
school course on animal rights. She is the founder and director of PNC
Inc., a non-profit animal rights foundation, which organized the first
international conference on contraception in wildlife, and which also
initiated and funded the first study on fertility control in deer using
the immunocontraceptive agent PZP.]

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